Polk County will start enrolling eligible women for a national study on children's health next month. The National Children's Study is the nation's largest long-term study on children's health. It aims to track 100,000 children from before birth to age 21. The Polk County Health Department on Wednesday announced enrollment will begin in mid-November. Women ages 18 to 49 at any stage of pregnancy and women who will likely become pregnant are invited to participate. Polk County is one of 105 locations selected nationwide.